1. Field of the Invention
This invention relates to a stainless steel sheet for welded structural components having excellent intergranular corrosion resistance and workability and which is, therefore, suitably used for vehicle structural components such as railway vehicles, automobiles, and buses, and civil engineering structural components which often undergo welding and bending and are required to have corrosion resistance.
2. Description of the Related Art
Structural components of vehicles, for example, railway vehicles, must have high corrosion resistance to maintain cosmetic appearance and to prevent a decrease in strength resulting from thickness reduction due to corrosion. Accordingly, austenitic stainless steel sheets, such as SUS301L and SUS304 specified in Japanese Industrial Standards (JIS), have been used for these structural components. The austenitic stainless steel sheets have excellent workability and toughness at the weld zone. However, when vehicles are manufactured, weld zones can be sensitized to cause intergranular corrosion, as shown in The 89th Corrosion Control Symposium Materials, “Case Study Method—Cases of Corrosion of Stainless Railway Vehicles”, pp. 84-91, Mar. 19, 1992, wherein “sensitized” means that, when a steel sheet is heated to high temperature, chromium carbide (Cr23C6) is produced at grain boundaries and, thus, a Cr depletion layer is formed around the chromium carbide. As for ferritic stainless steels, such as SUS430 specified in JIS, the grains become larger at the weld zone and, thus, the toughness at the weld zone decreases. In addition, chromium carbonitrides are precipitated in the coarse grain boundaries of the stainless steel to cause intergranular corrosion.
Martensitic stainless steel sheets for welded structural components, as epitomized by SUS410 specified in JIS, are suitably used to prevent intergranular corrosion because they are not significantly sensitized. However, since the martensitic stainless steels have a Cr content of about 12 mass percent, among the lowest in stainless steels and do not contain Ni and Mo, which enhance corrosion resistance, the corrosion resistance thereof is low and is not, therefore, satisfactory for use in parts exposed to observation.
Relating to these problems, Japanese Unexamined Patent Application Publication No. 11-302795 has disclosed an inexpensive stainless steel for general building structural components, having excellent corrosion resistance in housing conditions, weldability, and properties at the weld zone. The stainless steel is made by forming at least 50% by volume of martensitic structures in the welding heat affected zone (the region where the base material is not welded, but the hardness and structure thereof are changed by welding heat) and by refining the crystal grains to enhance the toughness. However, when martensitic structures are produced at the grain boundaries in the welding heat affected zone, the martensitic structures may be selectively corroded in some conditions to seriously degrade the intergranular corrosion resistance in the welding heat-affected zone. Thus, intergranular fracture may be caused by the corrosion. Highly corrosion-resistant martensitic stainless steels used for oil well pipes and pipelines generally contain 3 mass percent or more of Ni and, accordingly, have excellent corrosion resistance. The Ni, however, increases the resistance to anneal softening, so that the resulting structure after annealing is not a ferrite single-phase structure but contains martensitic structures, thereby increasing the strength to 800 MPa or more. Unfortunately, the highly corrosion-resistant martensitic stainless steels are not suitable for use in vehicle structural components and civil engineering structural components which often undergo bending.
No types of steel have been developed which has satisfactory resistance and workability in base material and satisfactory intergranular corrosion resistance and toughness at the weld zone.
It would accordingly be advantageous to provide a structural stainless steel sheet having remarkably enhanced intergranular corrosion resistance and excellent toughness at the welding heat affected zone, and further having, excellent workability with low strength and high elongation, and to provide a method for making the same.